Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
  • G01F1/68—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by using thermal effects
  • G01F1/684—Structural arrangements; Mounting of elements, e.g. in relation to fluid flow
  • G01F1/6845—Micromachined devices

Definitions

• the invention relates to a flow sensor, in particular for analyzing gas flows, according to the preamble of the main claim.
• Flow sensors currently available are often implemented as thin-film membrane sensors, the anemometric method being used to determine the respective flow. is used.
• thermocouples and a resistance heating conductor are provided on this self-supporting membrane.
• the recess under the self-supporting membrane serves to thermally isolate the thermocouples from the silicon substrate.
• a surface micromechanical structuring method for silicon is used to produce this integrated flow sensor, with a layer of porous silicon serving as the sacrificial layer.
• the thermocouples serving as sensor elements consist, for example, of polycrystalline p-type silicon / aluminum or polycrystalline n- Type silicon / polycrystalline p-type silicon.
• the resistance heating conductor is a strip of polycrystalline silicon.
• the so-called bulk micromechanics is known, in which the previously generated membrane is exposed from the back of the supporting body by different etching processes, for example wet-chemical or by a plasma etching process, by means of an opening which is etched there.
• porous silicon which is fundamentally known as a possible sacrificial layer for a surface micromechanical process, is an electrochemical reaction between hydrofluoric acid and silicon, in which a sponge-like structure is generated in the silicon.
• the wafer must be polarized anodically with respect to a hydrofluoric acid electrolyte.
• the resulting porous structure gives the silicon a large inner surface and therefore different chemical and physical properties than the surrounding bulk silicon.
• the reactivity of porous silicon significantly increased, which allows selective dissolution of the porous silicon compared to bulk silicon.
• Variously doped silicon substrates are suitable for the production of porous silicon, p-doped wafers usually being used.
• the structure size within the porous silicon is determined by the doping.
• porous silicon Various masking methods or masking layers as well as a so-called electrochemical etching stop are used in a locally defined production of porous silicon.
• a thin layer on the surface of the p-doped silicon substrate is doped into n-doped silicon as a masking layer, for example by implantation or diffusion of a dopant, so that the porous silicon can only be formed in the p-doped areas during subsequent electrochemical etching , Since the formation of porous silicon continues to be isotropic in this electrochemical etching process, the masking layer initially applied is completely under-etched, so that self-supporting structures are created.
• the object of the present invention was to implement a flow sensor in surface micromechanics with improved stability and improved thermal insulation of the actual sensor elements with respect to the supporting body. Another task was to develop a flow sensor which also allows angle-dependent detection of a flow, in particular a gas flow, and which enables an inexpensive and at the same time very flexible manufacturing process with regard to the layout of the flow sensor.
• the arrangement of the sensor components on an at least largely self-supporting web leading over a recess has the advantage over the arrangement on a self-supporting membrane that the thermal insulation of the sensor components is significantly improved and that at the same time several, for example on a network of webs arranged sensor components, can be arranged over a recess without these individual sensor components being in direct thermal contact with one another, ie here, too, there is very good thermal insulation both with respect to the supporting body and the individual sensor components from one another.
• a further advantage of the flow sensor according to the invention is the possibility of producing it in silicon using surface micromechanics, so that, for example, double-sided processing of a silicon wafer, which often causes undesired contamination, is not necessary.
• Another advantage of the flow sensor according to the invention lies in the simplified manufacture and further processing of the support body provided with the sensor components. This is especially true in the case where the poorly heat-conducting area is a porous silicon area or a porous silicon oxide area, since in this case contamination of a cavern cannot occur during further processing by means of customary construction and connection technology.
• porous silicon or porous silicon oxide as a heat resistance in the recess also significantly increases the stability of the entire flow sensor.
• a major advantage of the flow sensor according to the invention also lies in the extensive freedom in the layout of the flow sensor, i.e. the arrangement of the individual sensor components over the poorly heat-conducting area.
• At least one heating element for example a heating conductor in the form of a platinum resistance conductor track, is provided with which the sensor components can be heated or with which they can be brought to operating temperature.
• the sensor component has a conductor track, a thermistor, a thermocouple or a thermopile, wherein the design of the sensor component in the form of a
• Platinum resistance track has highlighted. However, other materials that can be deposited using thin-film technology, such as polysilicon, platinum or aluminum, are also suitable as the heating element or sensor component.
• a heating element and a plurality of sensor components are provided around the heating element or a central surface, these individual sensor components each of at least in regions of the support body are separated by a poorly heat-conducting area compared to the support body.
• a poorly heat-conducting area compared to the support body.
• Such an area is preferably assigned to each of the individual sensor components.
• those areas of the support body that are not occupied by one of the poorly heat-conducting areas are provided with a good heat-conducting cover layer, for example a silicon layer or platinum layer, which acts as a heat sink and the uniformity of the temperatures within serves the top layer.
• FIG. 1 shows a first method step for producing a flow sensor with a supporting body and a porous silicon area in section
• FIG. 2 shows a second method step with webs produced
• FIG. 3 shows a third method step with sensor elements arranged on regionally self-supporting webs.
• FIG. 4 explains the first method step of a second exemplary embodiment
• FIG. 5 shows a further method step of this exemplary embodiment, sensor components and a heating element having been produced
• FIG. 6 shows a top view of FIG. 5.
• FIG. 7 explains a third exemplary embodiment of a flow sensor for the angle-dependent detection of a gas flow.
• FIG. 1 first shows a p-doped silicon wafer as a supporting body 10, which is provided on the surface with a mask 12.
• the mask 12 is an n-doped silicon layer produced by redoping. It is further provided in FIG. 1 that an area with porous silicon 11 is produced in a surface area of the support body 10, the mask 12 being formed in the form of webs within the porous silicon area 11, which leads over the porous silicon area 11, and which are connected at their ends to the support body 10.
• FIG. 2 shows a further method step in which, starting from FIG. 1, the porous silicon region 11 has been converted into a region 11 ⁇ made of porous silicon oxide by an oxidation step.
• FIG. 3 then explains how a heating element 17 and sensor components 15 are applied to the surface of the mask 12 using thin-film technology.
• FIG. 3 further shows that after the application of the heating element 17 or the sensor components 15, the porous silicon oxide region 11 ⁇ has been removed, so that a recess 14 and at least regionally self-supporting webs 13 are formed, which have a thickness of less than 500 nm, in particular 100 nm to 200 nm, and on each of which the sensor components 15 and the heating element 17 run.
• FIG. 3 shows a further method step in which, starting from FIG. 1, the porous silicon region 11 has been converted into a region 11 ⁇ made of porous silicon oxide by an oxidation step.
• FIG. 3 then explains how a heating element 17 and sensor components 15 are applied to the surface of the mask 12 using thin-
• the heating element 17 is designed as a resistance conductor track running on a web 13, for example made of platinum. It is used to heat the sensor components 15, for example to keep them at an operating temperature of 100 ° C. or to bring the area heated by the heating element 15 to a defined excess temperature in relation to the area of the sensor components 15.
• the sensor components 15 are likewise platm resistance conductor tracks applied using thin-film technology.
• the deposition of the sensor components 15 or of the heating element 17 is preferably carried out physically / chemically, for example with the aid of a CVD method or by sputtering.
• the webs 13 are also made as thin as possible so that the sensor components 15 are thermally decoupled as well as possible from the supporting body 10 despite the design of the webs 13 made of silicon.
• known details on the manufacturing process, the deposition of the heating elements 17 or sensor components 15, as well as the formation of the recess 14 and details on the porosification of silicon reference is made to DE 100 30 352.8 or WO 98/50763, where these are described in detail ,
• FIGS. 4 to 6 explain an alternative embodiment to FIGS. 1 to 3 for the production of a flow sensor 5.
• a p-doped silicon wafer is used as the support body 10, which has a mask 12 made of n-doped Silicon is provided.
• the surface of the support body 10 is again provided with a porous, preferably nanoporous or mesoporous silicon region 11.
• Figure 5 shows then, how the porous silicon region 11 is converted by oxidation into a corresponding porous Siliziumoxid Scheme 11, and then as on the surface of the supporting body 10 or the porous Siliziumoxid Schemees 11 ⁇ a topcoat has been disposed sixteenth
• the cover layer 16 is a poorly heat-conducting layer, for example a silicon nitride layer. It serves to seal the porous silicon oxide region 11.
• the thickness of the cover layer 16 is, for example, 100 nm or more.
• the porous silicon region 11 or the porous silicon oxide region 11 is preferably produced with a degree of porosity greater than 60% in order to minimize the mass of remaining silicon and at the same time to ensure sufficient stability.
• the conversion of porous silicon into porous silicon oxide brings about a further reduction in the thermal conductivity, since good heat-conducting silicon is converted into poorly heat-conducting silicon oxide.
• silicon has a typical thermal conductivity of 150 W / km, silicon dioxide of 1.4 W / km, porous silicon of 1 to 2 W / km and oxidized porous silicon a typical thermal conductivity of 0.3 to 1.4 W / Km has. To this extent, it is preferable to ensure the best possible thermal insulation of the sensor devices 15 to generate a porous ⁇ Siliziumoxid Symposium. 11
• the covering layer 16 is preferably applied analogously to the application of the sensor components 15 or the heating elements 17, i.e. by a physico-chemical deposition process, for example a CVD process or sputtering.
• a physico-chemical deposition process for example a CVD process or sputtering.
• the thickness of the porous silicon oxide region 11 is chosen to be as large as possible, for example between 50 ⁇ m and 200 ⁇ m.
• the heating element 17 or the sensor components 15 are then deposited in the form of platinum resistance conductor tracks analogously to FIG.
• the heating element 17 again serves to heat the sensor components 15 or the entire cover layer 16.
• FIG. 6 shows a top view of FIG. 5, wherein it can be seen that two sensor components 15 have been produced side by side on the surface of the porous silicon oxide region 11, which are separated from the heating element 17. It should be emphasized that both the heating element 17 and the sensor elements 15 according to FIG. 6 are located on the cover layer 16, so that the porous silicon oxide region 11 ⁇ shown is actually not visible in plan view.
• FIG. 7 explains a third exemplary embodiment for the angle-dependent detection of a gas flow with the aid of the flow sensor 5.
• a porous silicon oxide region 11 is first of all made of p-doped silicon on the surface of the supporting body 10 in the manner already explained above with the aid of a corresponding masking 12 ⁇ has been generated, which in the specific example aims in a star shape at a central surface 19, which likewise consists of porous silicon. It is further provided that a cover layer 16 is then applied to the entire surface of the support body 10, which seals the porous silicon oxide regions 11.
• This cover layer 16 is not shown in Figure 7, but is completely analogous to Figure 5, are in the illustrated exemplary embodiment, finally, those loading rich of Tragkorpers 10 which are not occupied ⁇ of a porous silicon region 11, with a further, highly heat-conductive top layer 18, for example a silicon layer or platinum layer. This serves to prevent thermal crosstalk between the individual sensor components 15.
• the cover layer 16 according to FIG. 5 is dispensed with in the exemplary embodiment according to FIG. 7, since sealing of the porous silicon oxide regions 11 ⁇ is not absolutely necessary is required.
• this embodiment has disadvantages in terms of long-term stability.
• the cover layer 18 is preferably also applied in this case in order to precisely define the edges or boundaries of the heat sink produced.
• FIG. 7 finally show that on the surface of the supporting body 10 in the region of the porous silicon oxide areas 11 ⁇ total of eight sensor elements 15 in the form of U-shaped platinum resistance conductor tracks are applied.
• These sensor components 15 are thus designed completely analogously to FIG. 6, FIG. 3 or FIG. 5.
• a heating element 17 in the form of a platinum resistance conductor track is provided on the central surface 19 according to FIG. 7, to which an electrical current is applied via corresponding supply lines, so that the sensor components 15 can be heated via the heating element 17.
• the sensor components 15 are preferably arranged in a cross shape or star shape around the central heating element 17, so that an angle-dependent detection of a gas flow is possible with the aid of such a flow sensor 5.
• a possible shape of the heating element 17, as shown in FIG. 7, is a screw shape with a square base. It is obvious that there are a multitude of possibilities with regard to the layout of the flow sensors 5 explained. For example, starting from FIG. 3, it is readily possible to generate a network of webs 13, each of which is at least largely self-supporting over the recess 14, and on which there are sensor components 15 or also a plurality of heating elements 17, which are then, for example are arranged according to FIG.

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• Physics & Mathematics (AREA)
• Fluid Mechanics (AREA)
• General Physics & Mathematics (AREA)
• Measuring Volume Flow (AREA)

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• 2000
  • 2000-11-23 DE DE10058009A patent/DE10058009A1/de not_active Ceased
• 2001
  • 2001-10-11 WO PCT/DE2001/003906 patent/WO2002042723A1/de active Application Filing
  • 2001-10-11 JP JP2002544615A patent/JP2004514153A/ja active Pending
  • 2001-10-11 EP EP01997679A patent/EP1340052A1/de not_active Withdrawn
  • 2001-10-11 US US10/432,542 patent/US7040160B2/en not_active Expired - Lifetime

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